2 Answers
2

Usually, words are transcribed based on how they sound—or how it is imagined that they'd sound, since many words are borrowed into Japanese via print and aren't actually heard.

A Spanish-speaking friend offers /ko'βaksin/ as an IPA transcription of Cobaxin. Based on that, I think you can straightforwardly transcribe the Spanish version of the name in Japanese as コバクシン, replacing the consonants with their closest equivalents and adding the epenthetic vowel /u/.

The vowel /u/ is the most common epenthetic vowel in Japanese and is usually the default choice after consonants other than /t/ and /d/. After these two consonants, adding /o/ is most common.

These vowels are added when you need to break up an illegal consonant cluster. Consider the English word strength:

ストレングス /sutoreɴgusu/

Here, English strength /strɛŋθ/ has the consonant cluster /str/, which is not possible in Japanese, so vowels are inserted to break it up, resulting in /sutor/. (However, since the English has no /g/ sound in most dialects, we can guess that this transcription was made from an imagined pronunciation based on spelling. This is often the case.) As you can see, /u/ is added in each case, except after /t/, where /o/ is inserted instead.

Generally speaking, you can get pretty far by adding these two vowels whenever necessary. But giving a complete set of rules for transcription is probably beyond the scope of a single answer, I'm afraid; I'll focus on Cobaxin for the rest of this answer.

You indicate that in English the final vowel of Cobaxin is more like /e/ than /i/. If that's correct, then I think you could write コバクセン instead.

In your transcription, you've written z rather than s. If you think this is more accurate, then コバクジン or コバクゼン would be fine, again depending on the final vowel.

If you intend to write /si/ or /zi/, I would stick to the standard シ／ジ rather than write セィ／ゼィ or スィ／ズィ.

I don't think Japanese speakers usually distinguish [si] from [ɕi] or [zi] from [ʑi], so there isn't a lot of use in distinguishing a "straight" /s/ or /z/ from the allophone that usually occurs before /i/. If you did write it with the less usual katakana combinations, I don't think people would pronounce it the way you intend very often.